EFFECTS OF CELL-PERMEABLE CERAMIDES AND TUMOR-NECROSIS-FACTOR-ALPHA ON INSULIN SIGNALING AND GLUCOSE-UPTAKE IN 3T3-L1 ADIPOCYTES

Incubation of 3T3-L1 adipocytes with C-2(-) and C-6-ceramides (N-acety l- and N-hexanoylsphingosines) but not dihydro-C-2-ceramide increased 2-deoxyglucose uptake in the absence of insulin. This effect was inhib ited by PD 98059, LY 294002, and rapamycin, which block the activation of mitogen-activated protein kinase, phosphatidylinositol (PI) 3-kina se, and ribosomal S6 kinase, respectively. Long-term increases in PI 3 -kinase activity associated with insulin receptor substrate 1 (IRS-1) increased GLUT1 and GLUT4 concentrations in plasma membranes. This tog ether with increased GLUT1 (but not GLUT4) synthesis explains the incr ease in non-insulin-dependent glucose uptake. C-2-ceramide inhibited i nsulin-stimulated glucose uptake after 2 h by decreasing insulin-induc ed translocation of GLUT1 and GLUT4 to plasma membranes. This occurred when there was no increase in basal glucose uptake or decrease in act ivation of IRS-1 or PI 3-kinase. Incubation for 24 h with tumor necros is factor-alpha (TNF-alpha) but not C-2-ceramide decreased the concent ration and insulin-induced tyrosine phosphorylation of IRS-1 in this e xperimental system. Cell-permeable ceramides mimic some effects of TNF -alpha, especially in stimulating basal glucose uptake. We identified a site for inhibiting insulin-stimulated glucose uptake that is downst ream of PI 3-kinase. Our work provides further mechanisms for the effe cts of TNF-alpha and ceramides in increasing non-insulin-dependent, gl ucose uptake and decreasing insulin-stimulated uptake in vivo.